1. Field of the Invention
The present invention relates to a substrate cleaning apparatus which sequentially performs a liquid chemical process and a rinsing process in a single processing bath upon a substrate to be processed such as a semiconductor wafer, a glass substrate for a liquid crystal display device, a glass substrate for a photomask and the like.
2. Description of the Background Art
Heretofore, there has been known a substrate cleaning apparatus of what is called a one-bath type which sequentially performs a liquid chemical process and a rinsing process upon a substrate within a single processing bath in a substrate manufacturing process. The substrate cleaning apparatus of the one-bath type performs the liquid chemical process on a substrate by immersing the substrate in a liquid chemical stored in the processing bath and causing the liquid chemical to overflow through a top portion of the processing bath while supplying the liquid chemical to the processing bath through a bottom portion thereof. After the completion of the liquid chemical process for a predetermined period of time, the substrate cleaning apparatus of the one-bath type gradually replaces the liquid chemical in the processing bath with deionized water by supplying deionized water to the processing bath through the bottom portion thereof. Then, the substrate cleaning apparatus of the one-bath type performs the rinsing process for a predetermined period of time while causing the deionized water to overflow through the top portion of the processing bath.
Such a background art substrate cleaning apparatus measures the resistivity of the deionized water stored in the processing bath after a lapse of a predetermined time period since the start of the rinsing process. When the measured resistivity is not less than a predetermined threshold value, the substrate cleaning apparatus does not judge that impurities such as liquid chemical components remain in the processing bath, and completes the rinsing process successfully. When the measured resistivity is less than the predetermined threshold value, on the other hand, the substrate cleaning apparatus judges that impurities such as liquid chemical components remain in the processing bath, and issues an alarm to an operator. In this manner, the background art substrate cleaning apparatus controls the operation of completing the rinsing process, based on the resistivity of deionized water.
FIG. 4 is a graph showing an example of variations in the resistivity of liquids in the processing bath when the substrate cleaning apparatus of the one-bath type performs a cleaning process while replacing the liquids in the following order: dilute hydrofluoric acid (a liquid chemical), deionized water, an SC-1 (standard cleaning 1; NH4OH—H2O2—H2O) solution (a liquid chemical), deionized water, an SC-2 (standard cleaning 2; HC1—H2O2—H2O) solution (a liquid chemical), and deionized water. As shown in FIG. 4, the resistivity of liquids decreases during the liquid chemical process, and recovers during the rinsing process. The background art substrate cleaning apparatus completes the rinsing process successfully after recognizing that such a resistivity recovers to at least a predetermined threshold value. However, as shown in FIG. 4, the recovery curve of the resistivity during the rinsing process varies significantly depending on the type of chemical liquid used in the immediately preceding liquid chemical process. For example, the resistivity tends to increase less during the rinsing process subsequent to the liquid chemical process using the dilute hydrofluoric acid than during the rinsing process subsequent to the liquid chemical processes using the SC-1 solution and the SC-2 solution.
On the other hand, the background art substrate cleaning apparatus uses a recipe setting screen as illustrated in FIG. 5 to define the processing details of the liquid chemical process and the rinsing process. Whether to make a check of the resistivity at the completion of each process or not is also defined on the recipe setting screen (an area A2 in FIG. 5), and the threshold value serving as a criterion of the check is specified as a single value in other parameter files. For this reason, the background art substrate cleaning apparatus cannot set the threshold value of the resistivity at different values depending on the type of chemical liquid used in the liquid chemical process despite the fact that the recovery curve of the resistivity varies depending on the type of chemical liquid as discussed above.
To prevent the decrease in yield and in device characteristics in a semiconductor device manufacturing process, the substrate cleaning apparatus of the one-bath type as discussed above is used to clean a semiconductor wafer, thereby removing particles and metallic impurities from the surface of the semiconductor wafer. There is, however, apprehension that the resistivity of liquid does not sufficiently recover during the rinsing process under circumstances where the threshold value of the resistivity cannot be set individually depending on the type of liquid chemical as described above. This might cause a liquid chemical component to remain on the surface of the semiconductor wafer after the rinsing process, whereby the liquid chemical component gives rise to the decrease in yield and in device characteristics, even when the liquid chemical process removes particles and metallic impurities from the surface of the semiconductor wafer. In particular, there is a danger that reliability problems such as a withstand voltage failure of a gate insulation film come up in the step of forming the gate insulation film and in the step of forming a capacitor.
In the step of forming the gate insulation film on the surface of the semiconductor wafer, for example, a cleaning process as described above (i.e., the cleaning process performed while the liquids are replaced in the following order: dilute hydrofluoric acid, deionized water, the SC-1 solution, deionized water, the SC-2 solution, and deionized water) is performed as pre-cleaning. In such a case, insufficient removal of the liquid chemical component during the rinsing process subsequent to the liquid chemical process might give rise to problems to be described below.
Dilute hydrofluoric acid is used mainly for the removal of a sacrificial oxide film. If a dilute hydrofluoric acid component remains after the rinsing process, there is a danger that the dilute hydrofluoric acid component deteriorates the roughness of a silicon surface on which the gate insulation film is to be formed. Additionally, the dilute hydrofluoric acid component leads to the expansion of crystal defects and pits in the silicon surface to roughen the film quality of the gate insulation film to be formed thereafter, which might result in the decrease in reliability such as the withstand voltage failure of the gate insulation film.
The SC-1 solution is used mainly for the removal of particles. If an ammonia component in the SC-1 solution remains after the rinsing process, there is a danger that the ammonia component deteriorates the roughness of the silicon surface on which the gate insulation film is to be formed. Additionally, the ammonia component leads to the expansion of crystal defects and pits in the silicon surface to roughen the film quality of the gate insulation film to be formed thereafter, which might result in the decrease in reliability such as the withstand voltage failure of the gate insulation film.
The SC-2 solution is used mainly for the removal of metallic impurities. If a chloride component in the SC-2 solution remains after the rinsing process, light metal such as calcium (Ca) is liable to adhere to the surface of the semiconductor wafer. Thus, the light metal adhering to the surface becomes particles, which might cause the withstand voltage failure of the gate insulation film.